1. Field of the Invention
The invention relates to a process for extruding a hollow section or the like from a billet which is introduced into the bore of a container and, by means of an extrusion stem, is fed in the direction of extrusion into a shape-forming cross-sectional opening in a die. Furthermore, the invention also relates to a device which is specially suited for that purpose.
2. Discussion of the Prior Art
During extrusion, a material which is in a ductile state--in the case of metals this includes materials such as non-ferrous metals, sintered metals or steel but in particular material in the form of an aluminium alloy--viz., a heated cast billet or rolled bar of material, is pressed in the direction of extrusion by an extrusion stem--or in the case of hydrostatic extrusion, by means of a fluid--out of a container through one or more shape-forming openings in a die. In the case of direct or forward extrusion the stem moves in the direction of the die opening, in the same direction as the resultant section. In indirect or backward extrusion the material is moved in the direction counter to that of the stem, through a die which is mounted on the hollow stem.
To produce hollow sections, so called hollow dies with a die plate are employed in the extrusion process, an example of this is described in DE 24 46 308 Al. The die plate is integrated in a part of the mandrel to provide the outer contour of the resultant section. In that case, in order to shape the inner contour, a mandrel--in the case of multi-chamber sections a plurality of mandrels--is arranged such that the mandrel projects into the die plate and beyond the shape-forming region. In such a process the ductile material is guided over inlets and into the extrusion tool in such a manner that the strands from the individual inlets flow together again under mandrel support arms--into a welding chamber--where they are welded together again. As the extrusion process proceeds further, the material or the aluminium alloy flows past the mandrel and the die opening, thereby adopting the prescribed hollow section shape. The inlets are always situated outside around the hollow space or spaces in the section; the aluminium alloy is introduced into the shape-forming region of the die from the outside, in particular via a plurality of inlets. If in the case of multi-chamber dies the inner regions of the hollow section can not be adequately supplied with metal from outside, then additional feeding inlets are provided in the inner part of the hollow section.
The main supply inlets are always situated around the outer contour of the hollow space in the section.
The size of hollow section that can be manufactured i.e. its maximum diameter of circumscribing circle is limited therefore by the diameter of the container and the size of the inlets arranged around the hollow space as well as the strength of the above mentioned mandrel support arms.
In general, the maximum section size that can be manufactured on conventional extrusion presses is limited by the size of the extrusion press, the diameter of the container used and the strength of the extrusion die. The above mentioned patent DE 24 46 308 and patent DE 28 12 690 of the applicant are concerned with the dimensioning of the latter for hollow section manufacture using large mandrel surface area. Also, the quality of the extrusion weld is influenced by the outer edge zone of the extrusion billet flowing into the outer inlets in the extrusion die, with the result that it is possibly necessary to machine away the outer skin of the billet before use. Furthermore, the shaping capacity and the service life of the extrusion tool are considerably reduced by the high load on the mandrel surface, by creep resulting from this high load, and by bending.
Attention must also be given to the fact that metal billets, especially billets of aluminium alloys, are covered with contaminants--for example residual lubricant--and with an oxide layer. In particular, oxide particles on the end and outer surface of the ingot may be extremely detrimental with respect to the structure of the section; the resultant zone of contaminating inclusions in the section is relatively long--depending on the shape of the section and the extrusion speed. Consequently, with increasing quality requirements, manufacturers are forced to scrap increasingly longer lengths of section. The result is diminished output and lower cost efficiency due to diminishing yield of section length.